Many big cities have an impenetrable cloud of visible brown air around them. It smells bad. Also, the increase in carbon dioxide has unwanted side effects. Let's come up with solutions.
We force-convect the air across a huge tunnel full of plants, who absorb the pollution. The cleaned air is then accelerated into the city.
Advantages: Cleans all pollutants, and decreases carbon footprint
Disadvantage: Plants need light. Also, must remove dead plants. Expensive maintenance. Plants edibility is questionable.
"Powers through even the thickest smoke!" So we have a large network of these, maintaining one per minute. Maintenance consists of unplugging the device, wiping the pollution off, then reassembling and plugging back in.
Disadvantage: Excessive energy usage, expensive, maintenance crew eventually drops dead of exhaustion or requires expensive multiple shifts, does nothing for carbon dioxide, cartloads of dirty, ruined cloths.
All air is inhaled by a vacuum system, then bubbled through a large body of water.
Disadvantage: Rapidly saturates, stops working
Not polluting in the first place
Cars, factories, campfires, cigarettes, and anything that in any way smells bad is hereby banned by the department of STFU. Pollution will be cleaned away after the next rainstorm.
Advantages: Cheap, Fines pay for everything and then some, carbon footprint eliminated
Disadvantages: Radically decreased quality of life, infinite whining, severe economic damage
By attaching extra equipment to factories, cars, etc, the most noxious part of their pollution does not reach the air
Advantages: Pollution reduced by 99%
Disadvantages: Carbon output unaffected. Who pays for the scrubbers? (Whoever does probably resents it.)
Better Energy sources
Fusion power, electric engines, and so on, allow for the same activity to occur without the smoke and carbon.
Advantages: Infinitely awesome, carbon footprint eliminated
Disadvantage: Requires technologies not yet invented
Trains and Buses and other high density people movers replace cars.
Advantages: Moves lots of people, easily retrofitted with better energy sources
Disadvantages: Does not necessarily run when a person needs to travel
Ummm...make a fan with fins miles long, point it at the ocean, and blast the foul air away from the city in an enormous gale.
Advantage: Extremely cool looking
Disadvantages: Excessive energy use, large chance of property damage, chance of people and pets being blown out to sea or otherwise injured. Chance of pollution-cloud blowing back into the city when the fan is turned off, ruining the whole exercise, excessively crazy
When it rains, the rain absorbs the pollution. We can send planes into the clouds with chemicals that cause it to rain. Yes, we can totally do this right now.
Disadvantage: But then you'd have to make it rain basically all the time. Wouldn't that be inconvenient, wet, and annoying? Also, all the pollution would wind up in the ocean, which would be bad.
One of these alone isn't going to solve the problem, and any one of them will cost quite a bit. But two or three of these combined should end the pollution problem in the big cities.